Diode lasers are used in many imaging applications as a convenient and low-cost radiation source. Material processing applications may make use of suitably coupled diode laser radiation to change the nature or character of a workpiece. Image recording and display systems may use laser diodes to provide illumination for an optical system.
In one particular imaging application, a monolithic array of laser diode emitters may be used to illuminate a multi-channel light valve. A light valve generally has a plurality of individually addressable modulator sites; each site producing a beam or channel of imagewise modulated light. An image is formed by selectively activating the channels while scanning them over an image receiver. For high quality imaging it is usually necessary that channels be uniform in their imaging characteristics, a requirement that presents a difficult challenge for system designers since the illumination from a laser diode is highly astigmatic with poor overall beam quality. Consequently optical systems for gathering and formatting the light output seek to overcome the inherent limitations of the diode laser output in order to produce useable illumination.
U.S. Pat. No. 5,517,359 (Gelbart) describes a method for imaging the radiation from a laser diode array having multiple emitters onto a linear light valve. The optical system superimposes the radiation line from each emitter at the plane of the light valve, thus forming a single combined illumination line. The superimposition provides some immunity from emitter failures (either partial of full). In the event of such a failure, while the output power is reduced, the uniformity of the line is not severely impacted.
Even with superimposed emitters, the uniformity of the individual emitter radiation profiles still has an impact on the overall uniformity of the line. A good laser diode array can have emitters that are more than 20% non-uniform in the slow axis. When the radiation from a plurality of emitters is combined, some of the non-uniformities may offset each other but commonly 10-15% non-uniformity remains. Some light valves can accommodate this non-uniformity by balancing the output from each channel by attenuating output from channels that are more strongly illuminated. This, however, represents wastage of up to 15% of the useful light output since it is not possible to amplify weak channels.
U.S. Pat. No. 6,137,631 (Moulin) describes a means for mixing the radiant energy from a plurality of emitters on a laser diode array. The mixing means comprises a plurality of reflecting surfaces placed at or downstream from a point where the laser radiation has been focused. The radiant energy entering the mixing means is subjected to multiple reflections, which makes the output distribution of the emerging radiant energy more uniform.
Laser diode arrays having nineteen or more 150 μm emitters are now available with total power output of around 50 W at near-infrared wavelengths. While efforts are constantly underway to provide higher power, material and fabrication techniques still limit the power that can be achieved for any given configuration. In order to provide illumination lines with total power in the region of 100 W, an optical system designer may only be left with the option of combining the radiation from a plurality of laser diode arrays. In so combining the radiation, care has to be taken to retain the brightness of the illumination. This imposes practical limitations on the designer.
U.S. Pat. No. 7,209,624 (Reynolds et al.) describes an illumination system in which the radiation from one or more laser arrays is directed into a light pipe. The light pipe mixes the individual radiation contributions from the laser arrays and forms a uniform illumination line. The pointing direction of each of the laser arrays is monitored and controlled to preserve the brightness of the composite illumination line.
U.S. Pat. No. 5,923,475 (Kurtz et al.) describes an illumination system for a laser printer comprised of a laser diode array, a cross array illumination optics, a laser lenslet array, a spatial light modulator, and a fly's eye integrator which illuminates the spatial light modulator with flooded uniform light. The design seeks to provide spatially and angularly homogenized uniform flood illumination of the spatial light modulator from a plurality of diode laser emitters.
The power emitted by individual diode lasers having risen as a result of technological development and it has become possible to employ ever fewer individual lasers or laser emitters to attain a given power level. While this is in general a good development, there is one aspect that complicates optical systems design. The problem is that the increased coherence-induced fine structure results when the radiation is combined for practical application, whether this is through the use of optical fiber structures, mirrors, fly eye structures or any other device that largely retains the coherence of the light. When such arrangements are employed in illuminators for imaging purposes, as in the preparation, for example, of a laser imaged lithographic plate, the fine structure may be imposed on the lithographic plate producing unwanted striations.
Thus, there exists a requirement for a means to reduce or remove the fine structure from such illumination systems.